Ceramic waveguide filters are well known in the art. In the electronics industry today, ceramic waveguide filters are typically designed using an "all pole" configuration in which all resonators are tuned to the passband frequencies. With this type of design, one way to increase the attenuation outside of the passband is to increase the number of resonators. The number of poles in a waveguide filter will determine important electrical characteristics such as passband insertion loss and stopband attenuation. The lengths and thickness of the resonant cavities, also known as resonant cells or resonant sections, will help to set the center frequency of the filter.
FIG. 1 shows a view of a prior art waveguide filter without extracted poles. In a conventional waveguide filter, resonators are spaced longitudinally and an electrical signal flows through successive resonators in series to form a passband. Waveguide filters are used for the same type of filtering applications as traditional dielectric blocks with through-hole resonators. One typical application for waveguide filters would be for use in base-station transceivers for cellular telephone networks.
In FIG. 1, the prior art waveguide filter 100 is made from a block of ceramic material, a top surface 102, a bottom surface 104, and side surfaces 106. The waveguide filter 100 also has longitudinally spaced cavities sections 108 which are separated and defined by notches 110. The waveguide filter 100 has an input and output 112 which consist of metallized blind holes on the top surface 102. All external surfaces of the waveguide filter 100, including the internal surfaces of the input and output 112, are coated with a conductive material. The waveguide filter 100 shows a dielectric block having five resonant sections, all longitudinally spaced in series.
Turning next to FIG. 2, a graph of the frequency response for the prior art ceramic waveguide filter of FIG. 1 is provided. This graph shows Attenuation (measured in dB) along the vertical axis and Frequency (measured in MHz) along the horizontal axis. On this graph, Attenuation values are between 0-100 dB and Frequency values are between 900-1000 MHz. These values are representative of just one application of the prior art waveguide filters. As this graph shows, when using a conventional waveguide filter design, there are no poles of attenuation located outside of the frequency passband of interest. This can restrict the design freedom of an engineer who builds systems using waveguide filters.
FIG. 3 shows an electrical schematic of the circuit for the prior art ceramic waveguide filter 100 of FIG. 1. Waveguide resonant structures 302 are connected to electrical ground and are separated by the inter-structure inductive couplings 304 which are created by the vertical slots 110 in FIG. 1. The electrical input and output 306 are coupled via capacitors 308 located at the end of the waveguide structures through the dielectric ceramic monoblock.
Unfortunately, the addition of resonators to increase the attenuation outside of the passband has the adverse effect of increasing the insertion loss as well as the overall dimensions of the filter. This is contrary to the trend in the industry which demands smaller components which are lighter and use less space inside of electronics equipment.
To address this problem, the present invention provides for a ceramic waveguide filter with extracted poles. With an extracted pole waveguide filter design, the number of in-band resonators can be reduced and one or more resonators can then be tuned outside the passband. The resonators which are tuned outside the passband can then be coupled to the electrical input and electrical output to provide increased attenuation at specific frequencies. As a result, with the present invention, it is possible to get enhanced attenuation of frequencies outside of the passband for a given size waveguide filter.
A ceramic waveguide filter with extracted poles which is achieved by strategic placement of the electrical input and output components and which provides more electrical attenuation at specific frequencies without increasing the overall size of the filter would be an improvement in the art.